This invention relates to self-contained, two-component, interactable and orally settable alginate systems which are particularly adapted for use in dental therapeutics.
Alginate compositions have long been used in dentistry as impression materials for making impressions in areas in which partial dentures are to be constructed, for fabricating study models in orthodontic treatment, for making primary impressions in endentulous mouths, and as corrective materials in secondary impressions of all types.
As distinguished from agar-based thermally reversible hydrocolloids which gel by lowering the temperature of the heated and fluidized material, alginate compositions gel by means of a chemical reaction. After the alginate gel is formed, it cannot be converted to a fluid condition or sol by physical means and, thus, the alginates are known as irreversible hydrocolloids. The use of alginates in dental therapeutics is reviewed in the text entitled Elements of Dental Materials, by Ralph Phillips at Chapter 9.
Alginate compositions designed for use in dental therapeutics are typically formulated as powders which are adapted to be mixed with water to form a viscous sol. The sol is carried into the mouth in a perforated impression tray where it forms an elastic gel through a series of chemical reactions. Following formation of the gel, the impression is removed from the mouth for use in the construction of dental forms.
The principal ingredients of a prior art, powdered alginate impression material are illustrated by the following formula: potassium alginate (12%), diatomaceous earth (74%), calcium sufate (dihydrate) (12%), and trisodium phosphate (2%). The significant ingredient in the formulation is soluble potassium alginate which is derived from sea kelp. When the powdered alginate formulation is mixed with water, the soluble alginate reacts with the calcium sulfate to produce the gel structure of an insoluble calcium alginate. Since this reaction must take place and go to completion in the mouth, it must be delayed until the aqueous composition is placed in the impression tray and carried to the mouth. In order to effect this delay and provide adequate working time, a reaction rate retarder such as trisodium phosphate is incorporated into the formulation. The suggested mechanism for the effectiveness of the reaction rate retarder is that the calcium sulfate will reach first with the trisodium phosphate before reacting with the soluble alginate and that as long as any trisodium phosphate is present, the gelling reaction between the soluble alginate and the calcium sulfate will be prevented. A filler such as diatomaceous earth is also incorporated into the formulation to increase the strength and stiffness of the gel and to provide a firm surface that is not tacky. The final structure of the gel is characterized as a brush-heap network of fibrils of calcium alginate which hold the excess water and filler.
The powdered alginate compositions of the prior art have certain inherent problems. One of the problems encountered is that the water used in preparing the gel contains dissolved minerals such as calcium and iron salts which alter the divalent metal concentration in the powder formulation. The presence of the additional divalent metal ions can cause a grainy mix and uneven gel formation and, in addition, can have an adverse effect on the control of set time. This type of problem is most severe in those regions where the water supply has a high mineral content; and this type of problem is also vexing in those regions where the mineral content of the water supply varies throughout the day.
A second problem encountered with the powdered alginate compositions of the prior art relates to the temperature of the water used in the preparation of the gel, since this temperature is a principal factor in determining gelation or set time. In general, gelation time of the alginate composition is inversely proportional to the temperature of the admixing water. Accordingly, by increasing the water temperature, the gelation time is reduced. Likewise, if fluidity or flow of the material is measured against the temperature of the mixing water, the same relationship occurs, namely, the duration of the flow is inversely proportional to the temperature of the water. Flow is an important characteristic of the material, since the impression material must be fluid enough to move into intimate contact with the tissue so as to provide an accurate impression. Alginate formulations are usually designed to set in 2-3 minutes when mixed with water having a temperature of 72.degree. F. However, the temperature of the supply water varies from day to day and during the day in many regions which makes flow characteristics and set time difficult to control.
Also, the stone die must be added promptly to the alginate impression of the prior art so as to minimize inaccuracy through impression shrinkage as a result of water evaporation.
In addition, shelf life has been a continuing problem with powdered alginate formulations which deteriorate rapidly at elevated temperatures, or in the presence of moisture, or under both conditions resulting in the aqueous alginate formulation either failing to set at all or setting much too rapidly. This type of problem is accentuated by the association of a small amount of water with the ingredients used in making up the formulation.
Accordingly, it would be advantageous to provide an alginate system where the alginate is separated from the divalent metal ions until the time of use, where the system is self-contained with respect to water and thereby avoids the problem associated with varying water temperature and dissolved minerals, and where the system is formulated with humectants and plasticizers for enhanced stability and impression accuracy.